ugetrlimit • man page

getrlimit, setrlimit, prlimit - get/set resource limits

ugetrlimit • man page

getrlimit, setrlimit, prlimit - get/set resource limits

ugetrlimit (2)

Leading comments

Copyright (c) 1992 Drew Eckhardt, March 28, 1992
and Copyright (c) 2002, 2004, 2005, 2008, 2010 Michael Kerrisk
%%%LICENSE_START(VERBATIM)
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed u...

(The comments found at the beginning of the groff file "man2/ugetrlimit.2".)

The soft limit is the value that the kernel enforces for the
corresponding resource.
The hard limit acts as a ceiling for the soft limit:
an unprivileged process may set only its soft limit to a value in the
range from 0 up to the hard limit, and (irreversibly) lower its hard limit.
A privileged process (under Linux: one with the
CAP_SYS_RESOURCE
capability) may make arbitrary changes to either limit value.

The value
RLIM_INFINITY
denotes no limit on a resource (both in the structure returned by
getrlimit()
and in the structure passed to
setrlimit()).

The
resource
argument must be one of:

RLIMIT_AS

This is the maximum size of the process's virtual memory
(address space).
The limit is specified in bytes, and is rounded down to the system page size.
This limit affects calls to
brk(2),
mmap(2),
and
mremap(2),
which fail with the error
ENOMEM
upon exceeding this limit.
Also automatic stack expansion will fail
(and generate a
SIGSEGV
that kills the process if no alternate stack
has been made available via
sigaltstack(2)).
Since the value is a long, on machines with a 32-bit long
either this limit is at most 2 GiB, or this resource is unlimited.

RLIMIT_CORE

This is the maximum size of a
core
file (see
core(5))
in bytes that the process may dump.
When 0 no core dump files are created.
When nonzero, larger dumps are truncated to this size.

RLIMIT_CPU

This is a limit, in seconds,
on the amount of CPU time that the process can consume.
When the process reaches the soft limit, it is sent a
SIGXCPU
signal.
The default action for this signal is to terminate the process.
However, the signal can be caught, and the handler can return control to
the main program.
If the process continues to consume CPU time, it will be sent
SIGXCPU
once per second until the hard limit is reached, at which time
it is sent
SIGKILL.
(This latter point describes Linux behavior.
Implementations vary in how they treat processes which continue to
consume CPU time after reaching the soft limit.
Portable applications that need to catch this signal should
perform an orderly termination upon first receipt of
SIGXCPU.)

RLIMIT_DATA

This is the maximum size
of the process's data segment (initialized data,
uninitialized data, and heap).
The limit is specified in bytes, and is rounded down to the system page size.
This limit affects calls to
brk(2),
sbrk(2),
and (since Linux 4.7)
mmap(2),
which fail with the error
ENOMEM
upon encountering the soft limit of this resource.

RLIMIT_FSIZE

This is the maximum size in bytes of files that the process may create.
Attempts to extend a file beyond this limit result in delivery of a
SIGXFSZ
signal.
By default, this signal terminates a process, but a process can
catch this signal instead, in which case the relevant system call (e.g.,
write(2),
truncate(2))
fails with the error
EFBIG.

RLIMIT_LOCKS (early Linux 2.4 only)

This is a limit on the combined number of
flock(2)
locks and
fcntl(2)
leases that this process may establish.

RLIMIT_MEMLOCK

This is the maximum number of bytes of memory that may be locked
into RAM.
This limit is in effect rounded down to the nearest multiple
of the system page size.
This limit affects
mlock(2),
mlockall(2),
and the
mmap(2)MAP_LOCKED
operation.
Since Linux 2.6.9, it also affects the
shmctl(2)SHM_LOCK
operation, where it sets a maximum on the total bytes in
shared memory segments (see
shmget(2))
that may be locked by the real user ID of the calling process.
The
shmctl(2)SHM_LOCK
locks are accounted for separately from the per-process memory
locks established by
mlock(2),
mlockall(2),
and
mmap(2)MAP_LOCKED;
a process can lock bytes up to this limit in each of these
two categories.

In Linux kernels before 2.6.9, this limit controlled the amount of
memory that could be locked by a privileged process.
Since Linux 2.6.9, no limits are placed on the amount of memory
that a privileged process may lock, and this limit instead governs
the amount of memory that an unprivileged process may lock.

RLIMIT_MSGQUEUE (since Linux 2.6.8)

This is a limit on the number of bytes that can be allocated
for POSIX message queues for the real user ID of the calling process.
This limit is enforced for
mq_open(3).
Each message queue that the user creates counts (until it is removed)
against this limit according to the formula:

where
attr
is the
mq_attr
structure specified as the fourth argument to
mq_open(3),
and the
msg_msg
and
posix_msg_tree_node
structures are kernel-internal structures.

The "overhead" addend in the formula accounts for overhead
bytes required by the implementation
and ensures that the user cannot
create an unlimited number of zero-length messages (such messages
nevertheless each consume some system memory for bookkeeping overhead).

RLIMIT_NICE (since Linux 2.6.12, but see BUGS below)

This specifies a ceiling to which the process's nice value can be raised using
setpriority(2)
or
nice(2).
The actual ceiling for the nice value is calculated as
20 - rlim_cur.
The useful range for this limit is thus from 1
(corresponding to a nice value of 19) to 40
(corresponding to a nice value of -20).
This unusual choice of range was necessary
because negative numbers cannot be specified
as resource limit values, since they typically have special meanings.
For example,
RLIM_INFINITY
typically is the same as -1.
For more detail on the nice value, see
sched(7).

RLIMIT_NOFILE

This specifies a value one greater than the maximum file descriptor number
that can be opened by this process.
Attempts
(open(2),
pipe(2),
dup(2),
etc.)
to exceed this limit yield the error
EMFILE.
(Historically, this limit was named
RLIMIT_OFILE
on BSD.)

Since Linux 4.5,
this limit also defines the maximum number of file descriptors that
an unprivileged process (one without the
CAP_SYS_RESOURCE
capability) may have "in flight" to other processes,
by being passed across UNIX domain sockets.
This limit applies to the
sendmsg(2)
system call.
For further details, see
unix(7).

RLIMIT_NPROC

This is the maximum number of processes
(or, more precisely on Linux, threads)
that can be created for the real user ID of the calling process.
Upon encountering this limit,
fork(2)
fails with the error
EAGAIN.
This limit is not enforced for processes that have either the
CAP_SYS_ADMIN
or the
CAP_SYS_RESOURCE
capability.

RLIMIT_RSS

This is a limit (in bytes) on the process's resident set
(the number of virtual pages resident in RAM).
This limit has effect only in Linux 2.4.x, x < 30, and there
affects only calls to
madvise(2)
specifying
MADV_WILLNEED.

This is a limit (in microseconds)
on the amount of CPU time that a process scheduled
under a real-time scheduling policy may consume without making a blocking
system call.
For the purpose of this limit,
each time a process makes a blocking system call,
the count of its consumed CPU time is reset to zero.
The CPU time count is not reset if the process continues trying to
use the CPU but is preempted, its time slice expires, or it calls
sched_yield(2).

Upon reaching the soft limit, the process is sent a
SIGXCPU
signal.
If the process catches or ignores this signal and
continues consuming CPU time, then
SIGXCPU
will be generated once each second until the hard limit is reached,
at which point the process is sent a
SIGKILL
signal.

The intended use of this limit is to stop a runaway
real-time process from locking up the system.

This is a limit on the number of signals
that may be queued for the real user ID of the calling process.
Both standard and real-time signals are counted for the purpose of
checking this limit.
However, the limit is enforced only for
sigqueue(3);
it is always possible to use
kill(2)
to queue one instance of any of the signals that are not already
queued to the process.

RLIMIT_STACK

This is the maximum size of the process stack, in bytes.
Upon reaching this limit, a
SIGSEGV
signal is generated.
To handle this signal, a process must employ an alternate signal stack
(sigaltstack(2)).

Since Linux 2.6.23,
this limit also determines the amount of space used for the process's
command-line arguments and environment variables; for details, see
execve(2).

prlimit()

The Linux-specific
prlimit()
system call combines and extends the functionality of
setrlimit()
and
getrlimit().
It can be used to both set and get the resource limits of an arbitrary process.

The
resource
argument has the same meaning as for
setrlimit()
and
getrlimit().

If the
new_limit
argument is a not NULL, then the
rlimit
structure to which it points is used to set new values for
the soft and hard limits for
resource.
If the
old_limit
argument is a not NULL, then a successful call to
prlimit()
places the previous soft and hard limits for
resource
in the
rlimit
structure pointed to by
old_limit.

The
pid
argument specifies the ID of the process on which the call is to operate.
If
pid
is 0, then the call applies to the calling process.
To set or get the resources of a process other than itself,
the caller must have the
CAP_SYS_RESOURCE
capability in the user namespace of the process
whose resource limits are being changed, or the
real, effective, and saved set user IDs of the target process
must match the real user ID of the caller
and
the real, effective, and saved set group IDs of the target process
must match the real group ID of the caller.

RETURN VALUE

On success, these system calls return 0.
On error, -1 is returned, and
errno
is set appropriately.

ERRORS

EFAULT

A pointer argument points to a location
outside the accessible address space.

EINVAL

The value specified in
resource
is not valid;
or, for
setrlimit()
or
prlimit():
rlim->rlim_cur
was greater than
rlim->rlim_max.

EPERM

An unprivileged process tried to raise the hard limit; the
CAP_SYS_RESOURCE
capability is required to do this.

(prlimit())
The calling process did not have permission to set limits
for the process specified by
pid.

ESRCH

Could not find a process with the ID specified in
pid.

VERSIONS

The
prlimit()
system call is available since Linux 2.6.36.
Library support is available since glibc 2.13.

ATTRIBUTES

For an explanation of the terms used in this section, see
attributes(7).

Interface

Attribute

Value

getrlimit(),
setrlimit(),
prlimit()

Thread safety

MT-Safe

CONFORMING TO

getrlimit(),
setrlimit():
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

prlimit():
Linux-specific.

RLIMIT_MEMLOCK
and
RLIMIT_NPROC
derive from BSD and are not specified in POSIX.1;
they are present on the BSDs and Linux, but on few other implementations.
RLIMIT_RSS
derives from BSD and is not specified in POSIX.1;
it is nevertheless present on most implementations.
RLIMIT_MSGQUEUE,
RLIMIT_NICE,
RLIMIT_RTPRIO,
RLIMIT_RTTIME,
and
RLIMIT_SIGPENDING
are Linux-specific.

NOTES

Lowering the soft limit for a resource below the process's
current consumption of that resource will succeed
(but will prevent the process from further increasing
its consumption of the resource).

One can set the resource limits of the shell using the built-in
ulimit
command
(limit
in
csh(1)).
The shell's resource limits are inherited by the processes that
it creates to execute commands.

Since Linux 2.6.24, the resource limits of any process can be inspected via
/proc/[pid]/limits;
see
proc(5).

Ancient systems provided a
vlimit()
function with a similar purpose to
setrlimit().
For backward compatibility, glibc also provides
vlimit().
All new applications should be written using
setrlimit().

C library/ kernel ABI differences

Since version 2.13, the glibc
getrlimit()
and
setrlimit()
wrapper functions no longer invoke the corresponding system calls,
but instead employ
prlimit(),
for the reasons described in BUGS.

The name of the glibc wrapper function is
prlimit();
the underlying system call is
prlimit64().

BUGS

In older Linux kernels, the
SIGXCPU
and
SIGKILL
signals delivered when a process encountered the soft and hard
RLIMIT_CPU
limits were delivered one (CPU) second later than they should have been.
This was fixed in kernel 2.6.8.

In 2.6.x kernels before 2.6.17, a
RLIMIT_CPU
limit of 0 is wrongly treated as "no limit" (like
RLIM_INFINITY).
Since Linux 2.6.17, setting a limit of 0 does have an effect,
but is actually treated as a limit of 1 second.

A kernel bug means that
RLIMIT_RTPRIO
does not work in kernel 2.6.12; the problem is fixed in kernel 2.6.13.

In kernel 2.6.12, there was an off-by-one mismatch
between the priority ranges returned by
getpriority(2)
and
RLIMIT_NICE.
This had the effect that the actual ceiling for the nice value
was calculated as
19 - rlim_cur.
This was fixed in kernel 2.6.13.

Since Linux 2.6.12,
if a process reaches its soft
RLIMIT_CPU
limit and has a handler installed for
SIGXCPU,
then, in addition to invoking the signal handler,
the kernel increases the soft limit by one second.
This behavior repeats if the process continues to consume CPU time,
until the hard limit is reached,
at which point the process is killed.
Other implementations
do not change the
RLIMIT_CPU
soft limit in this manner,
and the Linux behavior is probably not standards conformant;
portable applications should avoid relying on this Linux-specific behavior.
The Linux-specific
RLIMIT_RTTIME
limit exhibits the same behavior when the soft limit is encountered.

Kernels before 2.4.22 did not diagnose the error
EINVAL
for
setrlimit()
when
rlim->rlim_cur
was greater than
rlim->rlim_max.

Representation of large resource limit values on 32-bit platforms

The glibc
getrlimit()
and
setrlimit()
wrapper functions use a 64-bit
rlim_t
data type, even on 32-bit platforms.
However, the
rlim_t
data type used in the
getrlimit()
and
setrlimit()
system calls is a (32-bit)
unsigned long.
Furthermore, in Linux versions before 2.6.36,
the kernel represents resource limits on 32-bit platforms as
unsigned long.
However, a 32-bit data type is not wide enough.
The most pertinent limit here is
RLIMIT_FSIZE,
which specifies the maximum size to which a file can grow:
to be useful, this limit must be represented using a type
that is as wide as the type used to
represent file offsets---that is, as wide as a 64-bit
off_t
(assuming a program compiled with
_FILE_OFFSET_BITS=64).

To work around this kernel limitation,
if a program tried to set a resource limit to a value larger than
can be represented in a 32-bit
unsigned long,
then the glibc
setrlimit()
wrapper function silently converted the limit value to
RLIM_INFINITY.
In other words, the requested resource limit setting was silently ignored.

This problem was addressed in Linux 2.6.36 with two principal changes:

*

the addition of a new kernel representation of resource limits that
uses 64 bits, even on 32-bit platforms;

*

the addition of the
prlimit()
system call, which employs 64-bit values for its resource limit arguments.

Since version 2.13,
glibc works around the limitations of the
getrlimit()
and
setrlimit()
system calls by implementing
setrlimit()
and
getrlimit()
as wrapper functions that call
prlimit().

COLOPHON

This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
www.kernel.org/doc/man-pages